The invention concerns an apparatus for decorating stiff objects by screen printing.
A form of apparatus for decorating, such as printing, inherently stiff objects using screen printing provides that, during the decorating or printing operation, the object to be printed upon is carried by an object carrier. The object on the carrier and at least one screen printing stencil are advanced synchronously along a transport path while a doctor co-operable with the stencil is movable in an opposite relationship to the direction of movement of the object and the stencil. Thus, an apparatus of that nature, which is to be found for example in German published patent application DE-OS 24 02 386, provides that the carrier and the object carried thereby are moved during the transport phase and also during the printing phase on a double chain which circulates in a vertical plane at a constant speed.
Although that apparatus represented a significant advance, in particular in terms of its output and efficiency, it nonetheless requires special items of equipment for introducing the objects into the continuously moving carriers intended to carry them, and for again removing the decorated objects from the carriers. Furthermore, in many cases it was found that that apparatus was not able to satisfy the requirements, which are usually imposed nowadays in terms of the quality of the decoration or printed image produced on the respective objects. It seems that this is due in particular to the fact that the double chain carrying the carriers and the objects thereon involves a certain play, even if minimal, which imposes certain limitations on accuracy of the alignment between the object and the screen printing stencil, such alignment being crucial to the quality of the print image.
Consideration may also be given to U.S. Pat. No. 6,082,256, disclosing an apparatus for printing on objects, in which holders, which pass along fixed guides, for carrying the objects are transported by a screw along a transport path defined by the guides. Using a screw as the drive in this case permits such accurate alignment of the objects in relation to the printing station where printing is applied to the objects that, when using rotary printing, it is possible to achieve in each respective printing station precise synchronization between the constant peripheral speed of the printing cylinder, on the one hand, and the linear speed of the object to be printed upon, which is constant in the region of the respective printing station, on the other hand.
The apparatus of U.S. Pat. 6,082,256 is also equipped with screen printing stations. It will be noted, however, that the operation of printing on the objects, which in this case involve CDs, is effected when they are stationary and the doctor is arranged to be transversely displaceable with respect to the direction of transport movement of the objects. Therefore, alignment of the stopped object with the screen printing stencil, which is also stopped, does not constitute any problems here. That mode of operation admittedly means that the screen printing stencil takes up a small amount of space, but in this case the residence time of each respective object in the screen printing stations is markedly longer than the residence time in the rotary printing stations. This means that the output of the apparatus generally is governed by the residence time of each respective object in the station in which the printing operation requires the longest period of time. Accordingly, it is not possible to make full use of the efficiency of the rotary printing stations, as the residence time of the respective objects in the screen printing stations is the factor that determines the output and thus the efficiency of the apparatus.